An environment compliance officer gunned down in northern NSW was ambushed on a public road, police say.

Police believe the accused killer, 79-year-old wheat farmer Ian Turnbull, pulled up unannounced beside victim Glen Turner and a colleague from the NSW Office of Environment and Heritage as they were inspecting an unrelated site on the side of a rural road in Croppa Creek, near Moree, last Tuesday afternoon, according to Fairfax Media.

Turnbull is then accused of opening fire on the pair with his rifle.

The police version of events throws into disarray claims that Turnbull was pushed to the edge by repeated harassment.

Mr Turner, a 51-year-old father-of-two, was not even on the Turnbull family's property at the time of the shooting and was not serving a notice for illegal land clearing, as Turnbull's family has previously claimed.

The victim died from a bullet in the back.

It was not yet known why Turnbull drove to the road or why he stopped.